Carbyne complexes of the group 6 metals containing 1,4,7-triazacyclononane and its 1,4,7-trimethyl derivative

Interaction of Cl(CO)₂py₂MCPh (M=Mo, W) with 1,4,7-trimethyl-1,4,7-triazacyclononane (Me₃TACN) in THF, followed by metathesis using NaBPh₄ in aqueous medium, readily afford [(Me₃TACN)M(CO)₂(CPh)]BPh₄ (M=Mo (1a), W (2)). [(TACN)Mo(CO)₂(CPh)]BPh₄ (1b) is similarly prepared using the molybdenum precursor and 1,4,7-triazacyclononane (TACN). Complex 1a, 1b and 2 are stable in the presence of concentrated hydrochloric acid at room temperature. The crystal structures of 1a and 2 both show that the three nitrogen atoms of Me₃TACN are not equidistant from the metal centre as a consequence of the different trans influences of the carbyne and carbonyl groups. Complexes 1a, 1b and 2 exhibit intense absorption bands at 320-340 nm and weak absorptions in the 400-500 nm region, while excitation of 2 at 330 nm in acetonitrile gives an emission at 630 nm with a lifetime of 83 ns (Φ_em=1.6×10^-4) at room temperature. The cyclic voltammograms of 1a and 2 in acetonitrile consist of a quasi-reversible couple (-2.15 V vs. Cp₂Fe^0/+) and an irreversible wave (+0.77 V vs. Cp₂Fe^0/+). Both couples are assigned to ligand-centred processes. Reaction of Me₃TACN with Cl(CO)₂py₂CrCPh in THF resulted in cleavage of the carbyne moiety to yield a binuclear product.